1. Field of the Invention
The present invention relates to a vertical revolute joint robot having an offset wrist, and more particularly, to a robot of this kind which is capable of rapidly calculating joint angles and excellent in operation accuracy.
2. Description of the Related Art
A typical six-axis vertical revolute joint robot comprises a robot body having an arm which consists of three links and a wrist consisting of three links. In this robot, a base fixed in a robot operation space is coupled to a link disposed on the side facing the base and adjacent ones of the links are coupled to each other by means of revolute joints, respectively, so as to permit an end effector mounted to the wrist to assume its arbitrary position and orientation. In order to control the position and orientation of the end effector to perform various operations, the robot comprises a computer for calculating target values of respective joint angles of first to sixth revolute joints on the basis of target position and orientation of the end effector so as to attain the target position and orientation, and a servo system for controlling the respective joint angles to the target values (here, the numbers specifiying the links and the revolute joints increase in sequential order outward from the base-side).
In the case of a so-called in-line wrist having three axes thereof crossing each other at a single point, the respective joint angles are derived from the position and orientation of the end effector in such a manner that the joint angle of the first revolute joint (hereinafter, the joint angle of the i-th (i=1-6) revolute joint is referred to as i-th joint angle) is first calculated, which angle is represented as a function solely of the position and orientation of the end effector, and then the respective i-th joint angle, represented as a function of the joint angle(s) previously calculated, such as the first joint angle, is calculated in sequence.
On the contrary, in the case of an offset wrist having three axes thereof not crossing each other at a single point and being offset relative to the arm, the first joint angle is not given at a function solely of the position and orientation of the end effector, and hence changes in dependence even on the fourth joint angle. Thus, the first joint angle is first calculated based upon a temporal initial value of the fourth joint angle. Then, a procedure of calculating the second to fourth joint angles and the first joint angle based on the first to fourth joint angles is repeatedly carried out until the thus calculated fourth joint angle converges.
In order to move the end effector along a predetermined path, with its orientation controlled to a target orientation, the robot periodically calculates the position and orientation of the end effector. In this respect, a calculation interval (interpolation period) for calculating the position and orientation of the end effector must be shortened. However, in the robot having the offset wrist, tremendous arithmetic operations must be made for calculation of the joint angles, as mentioned above. Accordingly, a required arithmetic time period is prolonged, resulting in a prolonged interpolation period. This makes it difficult to improve operation accuracy.